1. Field of the Invention
The invention relates to an apparatus for checking glue application state of an object to which a glue has been applied at a bottom surface thereof.
2. Description of the Related Art
In gluing processes for sack machines and collators, gluing is carried out by means of a glue applicator nozzle or a glue applicator wheel. The gluing needs to be tested as to whether the gluing has been carried out properly.
One of methods of testing whether glue is certainly applied to an object is a method in which two electrodes are disposed at the opposite sides of a glued object, and a change in electrostatic capacity between the two electrodes are detected in dependence on presence or absence of a glue. In another method, a glue is colored, and whether a glue is present or absent is detected by a light reflected from the glue. Japanese Unexamined Patent Public Disclosure No. 60-99641 has suggested still another glue application checking method which includes the steps of inserting a carton to which a glue is applied between a transmitting electrode plate and a receiving electrode plate, transmitting and receiving high-frequency waves between the two electrodes, and monitoring glue application state in accordance with attenuation of the high-frequency waves received by the receiving electrode.
In the method utilizing electrostatic capacity, since the two electrodes are facing with a glued object being disposed therebetween, one of the electrodes faces to a surface of the object onto which a glue is applied. Thus, a glue may adhere to a surface of the electrode facing to the surface of the object onto which the glue is applied. If a glue is adhered to an electrode, the variation of detected electrostatic capacity is remarkably increased, and hence it is no longer possible to detect a change in the electrostatic capacity. In addition, the detected electrostatic capacity is also influenced by humidity, and thus it is quite difficult to stably detect whether a glue is present or absent. The optical method needs a glue to be colored. In the method suggested by Japanese Unexamined Patent Public Disclosure No. 60-99641 in which a variation in voltage of the receiving electrode caused by a glue dispensed between the transmitting and receiving electrodes is to be detected, since an object to be glued has to be passed between the two electrodes, a space between the electrodes has to be large, and hence it is difficult to increase detection accuracy about whether a glue is present or absent.
The assignee has already suggested in Japanese Unexamined Patent Public Disclosure No. 5-340892 that a glue applicator nozzle is used as an electrode, and one more electrode is disposed so that an object to be tested is situated between the glue applicator nozzle and the electrode, and also suggested in Japanese Patent Application No. 5-056235 (U.S. Pat. No. 5,322,566) that a glue applicator nozzle is used as an electrode, and one more electrode is disposed at the same side of the glue applicator nozzle with respect to an object to be tested. However, even if a distance between the electrodes is shortened by the above mentioned suggestion, a signal representing whether a glue is present or absent is still small, and in addition, a noise signal is still relatively large because of temperature variation, humidity variation, and variation of a position of metal material disposed in the vicinity of a glue applicator nozzle, and hence it was difficult to obtain with high accuracy the signal representing whether a glue is present or absent. In addition, even by the above mentioned methods, it is impossible to check glue application state of an object glued by using a widely used glue applicator wheel.